In recent years, a number of damage cases has increased in which confidential information, such as personal information (e.g., address book data), recorded in a mobile telephone, such as a mobile telephone or a personal computer, is leaked to the third party resulting from theft or loss of the mobile telephone. In addition, a mobile telephone to which a money payment function (e.g., FeliCa (Registered Trademark)) using wireless communication is added has been popular. The mechanisms for preventing the damage from the theft or loss are desired for such an apparatus.
As one of the mechanisms, there has been known a system in which a mobile telephone and an authentication apparatus are owned separately, and each of the mobile telephone and the authentication apparatus includes a wireless communication function and holds a regular communication with its counterpart, to always monitor a relative distance between the mobile telephone and the authentication apparatus. In such a system, the mobile telephone and the authentication apparatus perform the intermittent communication in synchronous with each other, and the distance to the counterpart apparatus is detected by detecting a received signal level or judging whether or not the received signal level reaches a level at which a received signal can be demodulated instead of detecting the received signal level. If it is judged that the distance is longer than a predetermined distance or the distance is too long to demodulate the received signal, the function of the mobile telephone is stopped (e.g., the payment function is stopped). Furthermore, the authentication apparatus sounds an alarm by pushing a buzzer or the like, and then, this leads to prevention of the theft or loss (See, for example, Patent Document 1).
Patent Document 1: Japanese patent laid-open publication No. JP-10-041936-A.